The present invention relates to a wavelength-division multiplexing (WDM) telecommunication system using dispersion-shifted optical fibres, in which noise effects due to the so-called "Four Wave Mixing" (FWM) are avoided.
In the most recent telecommunication engineering, it is known to use optical fibres for sending optical signals of a predetermined frequency to carry information to be remotely communicated. It is also known that the optical signal sent through an optical fibre undergoes an attenuation during its travel, which necessitates amplification by means of respective amplifiers disposed at predetermined intervals along the line.
Optical amplifiers are conveniently used to achieve the required amplification. With optical amplifiers, the signal is amplified while remaining in an optical form, that is, without detection and regeneration of the signal. These optical amplifiers are based on the properties of a fluorescent dopant, such as erbium for example, which, when suitably excited by application of optical pumping energy, has a high emission in the wavelength band corresponding to the band of minimum attenuation of the light in the silica-based optical fibres.
The optical fibres used for transmission have a chromatic dispersion, which is due to the material that forms them and the refractive index profile, that varies with the wavelength of the transmitted signal and that goes to zero at a given value of the wavelength itself. This chromatic-dispersion phenomenon substantially consists of a widening in the duration of the pulses forming the signal during their travel through the fibre. This widening is due to the fact that the different chromatic components of each pulse are characterized each by its own wavelength and travel in the fibre at different speeds. Following this widening, temporally successive pulses that are well separated at the transmitter, can partially overlap at the receiver, after their travel through the fibre. They may even be no longer distinguishable as separate values, causing an error in the reception.
Fibres of the so-called "Step Index" (or SI) possess such optical features that the chromatic dispersion goes to zero at a wavelength value of about 1300 nm. Therefore, SI fibres at wavelengths close to 1500 nm, which is used for telecommunication, have an important value of chromatic dispersion capable of constituting a limit to the transmission speed. That is, SI fibres limit the possibility of sending a high number of successive pulses in a predetermined unit time without incurring errors at the receiver.
Also known are the so-called dispersion shifted fibres or DS fibres in which the zero point of the chromatic dispersion is shifted. DS fibres substantially are fibres that have optical features suitably selected to bring the zero point of the chromatic dispersion to a wavelength value in the range of 1500 to 1600 nm, which is commonly used for telecommunications. Fibres of this type are defined in the ITU-T G.653 Recommendation of March 1993, in which the chromatic dispersion of the fibre is provided to nominally go to zero at a .lambda.o wavelength value of 1550 nm, with a tolerance of 50 nm. These fibres are available for example from Corning N.Y. (USA) under the trade name SMF/DS (Registered Trademark) and from Fibre Ottiche Sud S.p.A.(FOS), Battipaglia (IT) under the trade name SM DS. Fibres of the above type are also described in U.S. Pat. No. 4,715,679, U.S. Pat. No. 4,822,399, and U.S. Pat. No. 4,755,022, for example.
It is also known that the need to send increasing amounts of information over the same transmission line has led to the sending of more transmission channels over the same line by a so-called "Wavelength Division Multiplexing" (or WDM) process. According to this technique, more channels consisting of analog or digital signals are simultaneously sent over the line consisting of a single optical fibre, and the channels are distinguished from each other in that each of them is associated with its own wavelength in the employed transmission band. This technique enables the number of the transmitted pieces of information per unit time to be increased, where the pieces of information are distributed among several channels and the transmission speed on each channel is the same.
It has been found however, that a WDM transmission through dispersion-shifted single-mode optical fibres gives rise to an intermodulation phenomenon between the channels, known as "Four Wave Mixing" or FWM. This phenomenon arises, in general, when the presence of three optical signals in the fibre gives rise to a fourth signal which can overlap the others, thereby reducing the system performance. This phenomenon is described for example in JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 8, No. 9, September 1990, Pages 1402-408. The effect is due to non-linear third-order phenomena that can become very strong due to the high field intensity in the fibre core and at the high interaction lengths between the signals. In greater detail, the same publication points out that, for a particular optical fibre, the greatest generation efficiency of the fourth wave (that is the noise effect in the system) is reduced by increasing the differences between the signal frequencies, the chromatic dispersion or the transmission length, due to the increased phase shift between the signals. In the case in which the optical fibre is a low-chromatic dispersion fibre (the above described DS fibre, for example) and has a small efficient area of interaction between the optical frequencies (signal-mode fibre), the non-linearity resulting from generation of the fourth wave can become a limit to transmission, in that the intermodulation products can fall within the receiving band and give rise to a noise source. The solution proposed in the above mentioned publication for planning WDM systems consists of considering the distance between the wavelengths of the different channels and the signal powers.
U.S. Pat. No. 5,327,516 discloses an optical fibre for WDM telecommunication systems that has an absolute value of the mean chromatic dispersion at 1550 nm included in a range between 1.5 and 4 ps/(nm.km) and a slope of the lower dispersion curve at 0.15 ps/(nm.sup.2.km) in a fibre portion not shorter than 2.2 km. As made clear in the text (column 3, lines 1-5), these features of the optical fibre substantially introduce a small degree of linear dispersion which produces a phase shift between the optical channels sufficient to avoid the above described non-linearity effects.
JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 10, No. 11, November 1992, pages 1553-1561 reports the existence of another condition at which the efficiency of the fourth-wave generation becomes maximum. The article points out that the condition occurs when two of the three optical carriers have wavelengths symmetrical with each other with respect to a wavelength of zero dispersion, or when one optical carrier has the same wavelength as the wavelength that brings the dispersion to zero. The same publication mentions the fact that, for disturbance in the line created during the fibre manufacturing process, the wavelength value that brings the dispersion to zero fluctuates along the fibre length. From experiments conducted by varying the optical pump frequency about the zero-dispersion wavelength, together with a test signal at a fixed wavelength of 1557.7 nm, the author detected different peak values for 2.5 km long fibre portions drawn from a single preform. This means that the wavelength of zero chromatic dispersion in each of the tested fibres was different even when the fibres had been drawn from the same preform. The difference between the detected zero dispersion frequencies was in the order of 100 GHz (corresponding to 0.8 nm wavelength). With 10 km long fibres, a quick reduction in the FWM efficiency was observed with the increase of the distance between the channel frequencies. As a consequence of these results, this publication concludes that the manufacture of a uniform fibre is necessary in order that one may be able to operate in a wider wavelength range.
JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 1, No. 10, October 1993, pages 1616-1621 reports a method and a fibre arrangement capable of eliminating noises resulting from the non-linearity of the transmission system by using speed dispersion fibres of the "normal" group (D&lt;0) as the transmission fibre and by introducing a short fibre portion of "anomalous" dispersion (D&gt;0) in order to bring the dispersion to zero again over the whole length included between two repeaters. This publication takes into account the case of FWM phenomena between optical channels and amplified spontaneous emission in single-channel and very long-distance systems.